Volume 11, Issue 11                                                                                                    June 6, 2003


Vegetable Insects - Joanne Whalen, Extension  IPM Specialist;   jwhalen@udel.edu



Continue to scout all melons for aphids, cucumber beetles, thrips and spider mites. Economic levels of melon aphids can be found and cucumber beetle populations have also reached threshold levels. In many cases, beneficial insect activity has significantly increased and can eliminate aphid populations. The treatment threshold for aphids is 20% of the plants infested with at least 5 aphids per leaf. Actara, Fulfill, Lannate and Thiodan are the labeled on melons and will provide melon aphid control. The treatment threshold for cucumber beetles in watermelons is 2 per plant. Carefully check field margins, under plastic, and cracks in ground around the base of plants for beetles. Since cucumber beetles vector bacterial wilt in cantaloupes, treatments should be applied before adult beetles feed extensively on cotyledons and first true leaves. Actara, dimethoate, or a pyrethroid will provide cucumber beetle control.



In addition to thrips and corn borers, we have had reports of aphids feeding on peppers. A mixture of potato and green peach aphids can be found. Where Admire was used at planting, we are not seeing aphids. A treatment may be needed prior to fruit set if you find 1-2 aphids per leaf for at least 2 consecutive weeks and beneficial activity is low.



Continue to sample fields for Colorado potato beetle adults and larvae. The treatment threshold for adults is 25 beetles per 50 plants and 10% defoliation. Once larvae are detected, the threshold is 4 small larvae per plant or 1.5 large larvae per plant. If multiple life stages are present, reduce these thresholds by one-half. Actara, cryolite, Spintor or Provado will provide control. A corn borer spray will be needed when we reach 700-degree days (base 50). Currently we have only accumulated 504 degree-days. If we see an increase in temperatures, the first spray should be needed in the next 7-10 days. Be sure to check our website (http://www.udel.edu/IPM/traps/latestblt.html) for the most recent moth catches in your area. Ambush, Baythroid, Furadan, Penncap, Pounce or Spintor will provide control. If you are scouting for infested terminals, the first treatment should be applied when 20-25% of the terminals are infested. Furadan or Monitor will provide the best control if you are waiting until you see infested terminals.  Potato leafhoppers populations remain light. Controls should be applied if you find ½ to one adult per sweep and/or one nymph per every 10 leaves. Dimethoate, a pyrethroid, Actara or Provado will provide control. In most cases, beneficial insects have helped to clean up potato aphid populations.


Snap Beans.

Continue to scout seedling stage fields for leafhopper and thrips activity. The thrips threshold is 5-6 per leaflet and the leafhopper threshold is 5 per sweep. If both insects are present, the threshold for each should be reduced by 1/3. Dimethoate, Lannate, Asana, Capture, or Warrior will provide control of both insect pests. As soon as the earliest planted fields begin to bloom, Orthene should be applied for corn borer control, especially if trap catches exceed 2 per night.


Sweet Corn.

Corn borer whorl infestations remain light - 5-10% infested plants. A treatment should be applied if 15% of the plants are infested. The best timing for a treatment is just as the tassels are emerging from the whorls. In recent years, Baythroid, Mustang, Penncap or Warrior have provided effective control. If corn was planted under plastic, the first silk sprays will be needed as soon as ear shanks are visible. Treatment will be needed on a 6-day schedule throughout the state.


We are also seeing cereal leaf beetle adults in seedling and whorl stage corn. A treatment may be needed if you find 10 beetles per plant and 50% of the plants exhibit damage.




Vegetable Crop Diseases Bob Mulrooney Extension Plant Pathologist; bobmul@udel.edu


The rainy, cool weather pattern is certainly favorable for some foliar diseases. If late blight was present in seed producing areas it would be a real threat under these conditions, fortunately it is not. The high winds we experienced this past week or any other type of damage to the foliage such as hail can damage leaves and open them for infection by secondary fungi such as Botrytis or Alternaria alternata or others. So protecting the crop with fungicides such as mancozeb or chlorothalonil at this point is beneficial. If pink rot control is important and you did not treat at planting, foliar applications of either Ridomil Gold MZ or Ridomil Gold/Bravo, or Flouronil when tubers are nickel-sized is suggested.  A second application should be made 14 days later.  Conditions are not favorable for early blight, but if the crop runs out of nitrogen early because of the amount of rain we have had, fungicides for early blight control may be needed earlier than normal. Quadris, Gem or Headline would be the most effective for early blight, and black dot applied at the end of flowering and repeated 14 days later.




Field Crops


Field Crop Insects - Joanne Whalen, Extension  IPM Specialist;   jwhalen@udel.edu



Potato leafhopper populations remain light. Continue to sample fields for potato leafhopper adults and nymphs. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa. Baythroid, dimethoate, Mustang or Warrior will provide effective control.


Field Corn.

Continue to sample no-till fields for true armyworm. We continue to find fields with above threshold levels. The treatment threshold for armyworms in corn is 25% infested plants with larvae less than one-inch long. A pyrethroid will provide effective control, but only if worms are less than one-inch long.


Unfortunately, slugs are still the number one pest in many no-till corn fields. The only available control options are liquid nitrogen applied at night when the plants are dry or the metaldehyde baits (Deadline MPs or TrailsEnd LG). A few days of sunny warm weather would help cure a lot of problems including the slugs!


Small Grains.

In most cases, the decision to treat cereal leaf beetle should have already been made and we can now find adults moving into surrounding fields. Aphid populations are still high in some fields throughout the state. Fields should be scouted for aphids through the soft dough stage. The treatment threshold for aphids is 20-25 per head with low beneficial activity (less than 1 per 50 aphids).  Beneficial insect activity including parasites, lady beetles and syrphid fly larvae, has increased in some fields. Be sure to watch for these predators since they can quickly reduce populations. Although grass sawfly and armyworm populations remain low, we have had a report of economic levels in a field in Snow Hill, MD so fields should still be scouted for these 2 insect pests.



With the wet field conditions and delayed soybean planting, a seed treatment for seed corn maggot should be considered in all fields. Materials containing permethrin, such as Kick-Start VP and Kernel Guard Supreme have provided effective control. Although very few fields are planted and emerged, be sure to watch for bean leaf beetle at emergence. A treatment for bean leaf beetle will be needed from plant emergence to the second trifoliate when you find 2 beetles per ft. row and a 25% stand reduction. A pyrethroid or dimethoate will provide effective control.



Field Crop Diseases Bob Mulrooney Extension Plant Pathologist; bobmul@udel.edu



Sepotria leafspot continues to be the most widespread disease currently. After looking at our variety plots in Georgetown where most of the wheat is in milky ripe to mealy ripe stage (Feekes GS 11.1-11.2), Septoria seems to be the most prevalent disease. Powdery mildew was present on a few varieties in the lower canopy, but was only in the upper canopy on ‘Roane’, which is now very susceptible. The Septoria complex of diseases is present in different parts of the state. In New Castle county, as reported last week, Septoria tritici (Speckled leaf blotch) seems to dominate while Septoria nodorum (Septoria leafspot and glume blotch) predominated at the Georgetown location. No scab was observed at Georgetown or Middletown this week. Drying weather will certainly help improve grain quality.



Spot blotch form of net blotch is the predominate disease in barley at Georgetown, while the classic net blotch symptoms predominate in Middletown. Some scald caused by the fungus Rhynchosporium secalis was present in Sussex County as well.


Here are more pictures of these diseases on wheat and barley. Last week some of the pictures were not properly identified.





Septoria nodorum is currently found in Sussex County and can cause glume blotch later.


Speckled leaf blotch caused by Septoria tritici is very similar and found in New Castle variety plots.




Net blotch. This classic symptom was seen in New Castle County.



Spot blotch form of net blotch on barley in Sussex County..



Septoria leafspot is present on the unifoliate leaves of early planted (April 22) soybeans. When the weather changes this disease should subside.


Septoria on Soybean


Grain Marketing Highlights - Carl German, Extension Crops Marketing Specialist; clgerman@udel.edu


Rain Makes Grain.

It is possible that the old adage "rain makes grain" could become the rallying cry in the commodities market this next week. The U.S. corn crop is now 95% planted and over 81% emerged. U.S. soybeans are 74% planted and nearly 40% emerged. The U.S. wheat harvest has begun in the Southwest with TX and OK reported to be ahead of schedule. The weather forecast is for a warming, drying trend over the corn belt for the next 6 to 10 days. IL and IA, the two largest corn producing states in the nation, both have corn crops rated in the 70% (+) good to excellent category. This is the first time since 1998 that both states’ crops were rated this well simultaneously at the end of May.


Weekly Exports Send Mixed Signals.

The weekly export sales report issued on June 5 was moderate for old crop soybean sales, mid-range to trade expectations at 111,800 MT. There were no new crop sales in the mix, all old crop. The trade was expecting to see some evidence of new crop sales.


Corn sales at 533,900 MT were just below the low side of trade expectations. This level is not likely to provide much impetus for positive price action between now and the release of the next USDA crop report. New crop corn sales totaled 43.1 MT.


Combined old/new crop U.S. wheat sales managed to meet trade expectations with 33.9 MT reported for old crop sales and 338,000 MT for next year.


Next Crop Report.

USDA's June crop report will be released on Wednesday, June 11th at 7:30 a.m. CST. Key questions will be "How much, if any will U.S. production estimates be increased? and "What will the effect be on ending stock estimates?" The primary focus of commodity traders right now is U.S. crop progress and demand.




Identifying Lightening Injury in Corn - Jim Lewis, Caroline County Ag Educator, University of Maryland; Jl139@umail.umd.edu , Richard Taylor, Extension Agronomist, University of Delaware; rtaylor@udel.edu


With the continuation of the current weather pattern, some growers, consultants, and agents are being called on to identify small, usually circular spots in corn fields.  These areas often show a small number of dead or dying plants in their center and are surrounded by many more plants that are lodged or twisted to some degree.  In Photo 1, you can see the roughly circular area with about 10 dead plants in the center.  Plants outside the initial kill zone showed injury at the soil line resulting in an inability of the stalk or leaf whorl to hold the plant upright (Photo 2).




Photo 1.  Dead plants in the center of a damaged area surrounded by plants showing damage at the soil level following a lightening strike (Photo by Jim Lewis).


Photo 2.  Corn plants outside the kill zone showing injury at the soil line resulting in severe lodging (Photo by Jim Lewis).


In this instance, the grower remembers lightening hitting the field narrowly missing an irrigation system parked a mere 70 feet away.  The strike likely occurred during a period of heavy rain that resulted in the area being ponded since the field is quite sandy and usually does not have standing water very long after rain stops.  It was estimated that about one-twenty fifth of an acre was affected by the lightening strike.  Photo 3 and 4 will give you a wider view of the damage to help you identify this type of damage when you see it.


Photo 3.  Lodged corn plants showing a roughly circular pattern in a field following a lightening strike (Photo by Jim Lewis).



Photo 4.  Lodged corn plants in a field following a lightening strike that luckily missed a nearby irrigation system (Photo by Jim Lewis).




Decision Criteria for Planting Corn Late - Richard W. Taylor, Extension Agronomist;



In a field meeting this past Tuesday at Middletown, DE, we discussed whether to switch away from planting more corn acreage.  The concensus was that for the most part we still have a few weeks left to plant corn before having to make the switch away from corn.  In part, this can be attributed to the fact that up to now we have accumulated many fewer growing degree days than is normal for Delmarva.  The cool and sometimes cold wet weather means that instead of much of the crop being at the stage when we would normally be worried about getting it all sidedressed with nitrogen (N), it is still mostly less than 8 inches tall.


There are some suggested practices if you do plant more corn in the next couple of weeks.  You should try to stick with hybrids that have a relative maturity of about 105 to 110 days or less if planting towards the end of the window.  Late planted corn often attracts more insect problems, so it is important to use a Bt hybrid to help control corn borer problems.  The insect problem is usually worse when there is a large difference in the size of the late planted and early planted corn.  Unless something changes or you are unable to plant until long after warm weather occurs, this year there may not be as big a difference between late-planted and early-planted corn.  This may lessen the potential damage from insects, but it will not eliminate the problem.


Another concern as you switch to earlier maturing hybrids is their disease resistance package.  Late-planted corn frequently shows more stress from corn diseases than early-planted corn, so always choose a hybrid with the best possible disease resistance package when planting it late.


In a recent article, Dr. Bob Nielsen from Purdue University indicated that when planting corn late the yield potential from early, medium, and late maturing hybrids is very similar.  However, he did indicate that there may be considerable difference among maturity groups for moisture content at harvest.  The earlier hybrids tended to have lower moisture content and therefore would reduce drying costs.


Another question raised was whether to boost or reduce the seeding rate.  If you could predict the weather and were assured it would rain all summer, then you might choose an irrigated population as target.  Since I don’t know, my suggestion is to keep the same seeding rate as you normally use for your corn plantings.


When should you consider switching?  My advice is that some time between June 20-25th you should seriously consider moving to soybeans assuming your herbicide program does not preclude rotating to soybeans.


This does bring up another question.  If you have already applied your herbicide program or if you have to come in to replant a field that has been sprayed with herbicides, you will be constrained as to what crop you can seed.  Be sure to take into account limitations such as this when making decisions.  Also, if you have applied manure or N fertilizer to a field that you switch away from corn and to soybeans, you should keep in mind some of the problems that can occur.  We have found that where manure has been applied and a soybean crop is planted, there can be a yield reduction if the supply of N from the manure runs out during early pod fill.  Where high soil N levels exist, nodulation of the soybeans is weak.  If the soil N supply suddenly becomes inadequate during flowering and early pod fill, the time lag between the need for N fixation and how quickly the nodules can form and begin to fully function is enough to cause limitations on yield potential.




Keep Close Watch on Corn for Nitrogen Stress - Richard W. Taylor, Extension Agronomist, University of Delaware; rtaylor@udel.edu


Recent intense rainfall in some areas of the state could lead to nitrogen (N) stress in emerged corn fields.  When soils become saturated with water, N that is in the nitrate form can undergo a biological process called denitrification.  In this process, certain soil anaerobic bacteria convert nitrate-N to reduced forms such as N2, NO, and N2O which bubble out of the soil in a gaseous state.  Generally, the process is temperature dependent with only 1 to 2 percent of nitrate-N denitrified per day (when the soil is saturated), if the temperature is less than 55˚ F, but 4 to 5 percent denitrified per (saturated) day if the soil temperature is above 65˚ F.


Most of us apply a small amount of N (20 to 30 lbs N/A) as a starter to keep the crop growing until sidedress time.  This year soil temperatures have remained lower than normal restricting the amount of N mineralized from the soil organic matter (SOM) which supplements what is applied as starter.  For corn that has been planted for a month or more, much of the N has been converted into the nitrate form of N even if it was originally applied as the urea, ammonium, or other reduced form of N.  Even at the 1 to 2 percent rate of denitrification, we are likely to be losing from ¼ to ½ pound of N per acre per day of saturated conditions.  Even when the entire field is not saturated, there will be areas where saturated conditions exist on the small or micro-scale.


On sandy soils, the leaching of N below the rooting zone is also of concern.  Because of the wet conditions and lack of sun to warm the deeper layers of soil, it is likely that root development has been limited up to this point in the season.  There is a strong possibility that much of the N applied up to this point has leached below the corn’s rooting zone.


During the next few weeks, carefully observe your corn fields to identify as soon as possible any symptoms of N deficiency stress in the corn.  If symptoms appear (usually they first appear on the oldest leaves), consider applying at least part of the remaining N requirement right away to get the crop growing again.  I would suggest that you consider adding only a portion of the total planned N rate if the crop is still very small.  If the weather suddenly warms up and we receive plenty of sunlight, a disorder called rapid growth syndrome or twisted whorl syndrome can occur.  Typically, it occurs when periods of slow corn development (from planting to the current date) are followed by a rapid change to a period of rapid corn development (warm, sunny weather, plenty of soil moisture and excellent soil nutrient status).  It can affect entire fields or individual plants, but usually doesn’t show up until after corn plants are showing 5 to 6 leaf collars.  Often the whorls of the affected plants will appear tightly twisted and can bend over at almost right angles.  Plants usually have difficulty breaking out of the twisted whorl and when they do the new leaves will be very light in color or almost yellow. 


Limiting N additions, or where irrigation is available, applying N on an as needed basis can prevent the abrupt change from slow development to very rapid development.  You may see some differences among hybrids in their susceptibility to this problem.




Update on Last Week’s Soil-Applied Herbicide Table - Mark VanGessel, Extension Weed Specialist, mjv@udel.edu


Last week I included a table for how late soil-applied herbicides can be applied.  The labels for Python and Hornet have been changed lately and they can be applied to corn up to 20 inches tall.  Also, keep in mind, just because a herbicide is labeled for use on emerged corn, it does not mean that the herbicide will provide residual control.  Refer to pesticide labels or Delaware Pesticide Guides for level of control you can expect from a herbicide on emerged weeds.




Burndown Control for No-Till Soybeans - Mark VanGessel, Extension Weed Specialist, mjv@udel.edu


Due to the rains, many no-till soybean fields have not been sprayed.  Note that Gramoxone labels list maximum weed size of 6 inches, Roundup labels list winter annual weed sizes from 6 to 18 inches (maximum size differs for weed species and rate), and most 2,4-D labels do not list maximum weed size.  The long and short of it is that most of these fields have weeds larger than the labels recommend and it will be difficult to achieve good control.




Careful of Surfactant Use in this Weather - Mark VanGessel, Extension Weed Specialist, mjv@udel.edu


Due to overcast skies and ample moisture plants have very thin cuticles which may result in increased injury with postemergence herbicides.  Nitrogen additives are most likely to cause crop injury with this weather.  Also, consider using non-ionic surfactant rather than crop oils to reduce the risk of injury.  University of Delaware data supports use of non-ionic surfactants over crop oil concentrates because it provides similar levels of weed control as crop oils with less risk of injury.  This has been true in weather patterns, we are currently experiencing, as well as in dry weather




Rainfastness and Maximum Corn Size for Postemergence Herbicides - Mark VanGessel, Extension Weed Specialist, mjv@udel.edu


Rainfastness is number of hours needed between time of application and rainfall or irrigation to ensure sufficient absorption in the plant.


Broadcast applications refer to an over the top application and directed refers to use of special spray equipment to direct the spray and avoiding the spray coming in contact with the whorl of the corn.   When corn height and collar number are given, base decision on whichever feature is first attained.







interval (hr)

Maximum corn size



broadcast:  6 collars or 20 in.

directed: 10 collars or 36 in.



broadcast: up to 8 collars

directed: when necessary



12 inches tall




more than ½ pt/A:

     broadcast: 5 lvs or 8 in.

     directed: 36 inches tall

½ pt/A or less:  36 inches tall



No restrictions listed



broadcast: min- 4 inches tall

     max- 20 in. tall or 6 collar

directed: pre-tassel






30 inches tall or 8 collars

2,4-D Amine


broadcast: 8 inches tall

directed: pre-tassel

2,4-D Ester


broadcast: 8 inches tall

directed: pre-tassel



directed only: 12 inches tall do not apply 3 weeks before tasseling

Harmony GT


1 - 4 collars or 12 inches tall



broadcast: 24 inches tall or 7 collars

directed: 20 to 36 inches tall



broadcast: 16” tall or V5

directed: 16-36” tall



broadcast: 48 inches tall

directed: when necessary


Poast Plus


broadcast: emergence to start of pollen shed

directed: when necessary (depending on corn canopy and weed ht.)



broadcast: 2- to 10-lf collars

directed: when necessary; when corn leaves interfere w/ spray

Roundup products


up to 30 inches or 8 collars



24 inches tall



up to 8 collars



until 68 days pre-harvest






2 collars or 6 inches tall

Basis Gold


5 collars or 12 inches tall

Celebrity Plus


broadcast: 4 to 24 inches tall



6 oz rate:  4 to 10 inches  tall

4 oz rate:  up to 24 in.  tall

directed:  4 oz up to 36 inches tall



broadcast: min- 4 in. tall

     max- 20 in. tall or 6 collar

directed: 20 to 30 inches tall

Field Master


do not apply to emerged corn

Hornet WDG


broadcast: 20 inches tall or 6 collars

directed: 20 to 36 inches tall



12 inches tall

Liberty ATZ


12 inches tall



broadcast: 12 inches tall

directed: 20 inches tall



broadcast:  5-lf stage or 8 inches tall



broadcast: min- 4 inches tall

     max- 20 in. tall or 6 collar

directed: 20 to 30 inches tall

Ready Master ATZ


emergence until 12 inches tall



broadcast: 8 inches tall

directed: 12 inches tall

        or if rate >2 pts



broadcast: 20 inches tall or 6 collars (min. 4 in. tall)

directed: 20 to 24 inches tall (before tassel emerg.) 



less than 20 inches or 6 collars



spike through 36 inches tall


            an “-“ means no information on label



               Weather Summary


Weeks of May 30 to June 4, 2003


0.09 inches: May 31

0.03 inches: June 1

0.06 inches: June 3

0.37 inches: June 4

Readings taken for the previous 24 hours at 8 a.m.

Air Temperature:

Highs Ranged from 78°F on May 30 to 64°F on

June 1

Lows Ranged from 61°F on May 31 to 49°F on June 2

Soil Temperature:

66°F average for the week.

(Soil temperature taken at a 2 inch depth, under sod)


Web Address for the U of D Research & Education Center:




Compiled and Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops



Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State University and the United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.  It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, sex, disability, age or national origin.

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